It is often desirable in radio antenna transmitting or receiving systems to separate a radio frequency signal into separate frequency bands. In some applications, a so-called quasi-optical diplexer has been employed in the past to separate coincident radio signals of different frequency bands. For example, one such use of a quasi-optical diplexer is disclosed in "Imaging Reflector Arrangements to Form a Scanning Beam Using a Small Array," C. Dragone and M. J. Gans, The Bell System Technical Journal, Volume 58, No. 2, February, 1979. In this publication, a frequency diplexer is positioned between a transmit array and an imaging reflector. A receive array is positioned on one side of the diplexer, opposite that of the transmit array. Signals in the transmit band pass from the transmit array through the diplexer to the imaging reflector. The diplexer is reflective of signals in the recieve band, consequently, a signal in the receive band which is incident on the diplexer is reflected onto the receive array. The arrangement discussed immediately above is particularly compact and therefore finds useful application in satellite antenna systems.
Resonant-grid, quasi-optical diplexers of various configurations are disclosed in "Resonant-Grid Quasi-Optical Diplexers," J. A. Arnaud and F. A. Pelow, The Bell System Technical Journal, Volume 54, No. 2, February, 1975, and "On the Theory of Self-Resonant Grids," I. Anderson, The Bell System Technical Journal, Volume 54, No. 10, December, 1975. As discussed in these two latter-mentioned articles, in many millimeter-wave systems associated with communication satellite antennas or Hertzian cables, quasi-optical filters and diplexers are quite useful. Because of their large areas, quasi-optical devices have large power-handling capability and the problem of multi-moding is, in a sense, avoided. The ohmic losses can be small, and the grids are easy to manufacture by photolithographic techniques. These articles disclose a number of single-grid and double-grid diplexers. Each of the grids includes grid elements of various configurations which effectively form either a capacitance or an inductance. A grid, regardless of its geometry or design, can be represented by circuit elements that are found empirically by fitting the measured response curve of the grid to one calculated from the equivalent circuit. The article "Resonant-Grid Quasi-Optical Diplexers" mentioned above discloses numerous grid patterns, including a grid arrangement having capacitive elements that resemble a so-called "Jerusalem cross". At the resonant frequency, the Jerusalem cross grid is perfectly reflecting and behaves as a plain sheet of copper.
The frequency transition for prior art, quasi-optical diplexers has not been particularly sharp and the difference in the reflectivity of the separate frequency bands has not been sufficiently great for some applications. Moreover, the width of the separate frequency bands has been less than desired for some applications. Finally, because of the relatively narrow separation of frequency bands in some systems, it has been necessary to employ multiple frequency select screens which must be carefully oriented relative to each other, whereas the use of a single screen would have been preferred.
The present invention overcomes the deficiencies of the prior art discussed above.